It has long been known that with acrylate pressure-sensitive adhesive (PSA) compositions in particular the generation of outstanding adhesion properties requires crosslinking. With rubbers, too, the adhesion properties are improved by crosslinking.
In the production of adhesive tapes, the use of radiation crosslinking by means of UV radiation or electron beam (EB) has particular advantages over chemical/thermal crosslinking methods. Radiation crosslinking is also used advantageously to produce double-sided adhesive tapes.
The depth distribution of the absorbed radiation dose in a product exposed to accelerated electrons is known for any given acceleration voltage. A number of authors have developed empirical functions for this (for example, Heger, beta-gamma 1, 20, 1990; Neuhaus-Steinmetz, RadTech Europe, Mediterraneo 1993).
If the maximum acceleration voltage of the electron beam unit is inadequate for sufficiently uniform irradiation through the product owing to its high weight per unit area, the literature, and company brochures, describe the possibility of irradiation from both sides, the acceleration voltage and radiation dose set for both sides being the same.
For products which consist of a coating to be crosslinked, which can, for example, be a pressure-sensitive adhesive, and of a radiation-degradable backing, such as, for example, paper, woven or nonwoven cellulose, and OPP films, the damage can be minimized by optimizing the acceleration voltage. In this case the backing receives a significantly lower average dose than does the coating, while the dose reduction in the coating remains within acceptable limits.
Such conditions are described, inter alia, in EP 0 453 254 B (Yarosso et al.) and in the paper accompanying a lecture given by Dr Karmann to the 7th Munich Adhesives and Finishing Seminar, 1982.
A double-sided adhesive tape consisting of a backing with the adhesive compositions to be crosslinked on both sides and with an antiadhesive release liner should receive a dose of not more than about 10 to 50 kGy when irradiated uniformly from one side with accelerated electrons; otherwise, the release liner must be changed owing to the unacceptable deterioration of the mechanical and antiadhesive properties. The maximum acceptable absorbed radiation dose is a function of the type of pressure-sensitive adhesive and type of release coating.
Even here, given suitable layer thicknesses, a reduction in the unwanted effects can be achieved by a skilful choice of the accelerating voltage if the radiation dose in the release liner has already dropped significantly. However, it should be noted here that the PSA layer facing the release liner must still receive a radiation dose sufficient for crosslinking.
If both sides of a double-sided adhesive tape comprising a backing with PSA compositions on both sides and an antiadhesive release liner is irradiated symmetrically, the latter receives the full radiation dose. This also applies to those tapes known as transfer tapes, in which the pressure-sensitive adhesive to be crosslinked is coated without a further backing onto a release liner.
From the remarks above it is clear that the necessary expenditure for the crosslinking of double-sided adhesive tapes with electron beams becomes considerable when the radiation doses required to crosslink the PSA layer are so high that the mechanical and antiadhesive properties of the release liner are damaged to an unacceptable extent, since in that case the release liner must be changed for a new, unexposed liner by re-covering.
The object of the invention is to allow advantageous manufacture of double-sided adhesive tapes or, generally, double-sidedly coated backing materials, with radiation crosslinking using accelerated electrons or UV radiation.
This object is achieved by a process as set out in the main claim. The subclaims describe advantageous embodiments of the process and also a second embodiment of the process.
The invention accordingly provides a process for the radiation crosslinking of double-sided adhesive tapes, in which a backing material coated on both sides with adhesives is irradiated asymmetrically from both sides with different doses in an irradiation means.
In one advantageous embodiment, the process for the radiation crosslinking of double-sided adhesive tapes consists of the following steps:
a) coating a backing material with an adhesive A,
b) EB-crosslinking the adhesive A/backing subunit on the composition side with a dose A and acceleration voltage A set on the EB unit,
c) lining the adhesive A with a release liner,
d) coating the second side of the backing material with the adhesive B, and
e) EB-irradiating the assembly on the exposed side of the adhesive composition B with a dose B and acceleration voltage B set on the EB unit, the side carrying the release liner being guided through the EB unit preferably on a cooling roll and the dose A and the dose B and/or the acceleration voltage A and the acceleration voltage B having different values.
There is no need to change the release liner, and no re-covering operation is necessary. The steps of the process can be performed in one pass. If considered necessary, it is also possible to carry out in-line pretreatment and an additional levelling of the backing, i.e. a smoothing of the backing by heating owing to an absence of flatness in the backing (warping, for example).
To this end, first of all, in the case of EB crosslinking, the acceleration voltage and dose for the second irradiation that are to be set on the EB unit are calculated as a function of the individual thicknesses of the layers of the assembly, with total doses of up to 80 kGy or more in the composition layers, preferably with the aid of a computer program, such that
a) the dose on the exposed side of the release liner remains less than 40 kGy, preferably less than 10 kGy,
b) the dose at the release liner/adhesive composition A interface remains less than 50 kGy, preferably less than 15 kGy,
c) the surface dose in the adhesive composition B re main s less than (target dose +25%), preferably less than (target dose +15 %), and
d) the dose at the backing/adhesive composition B interface remains greater than (target dosexe2x80x9425%), preferably greater than (target dosexe2x80x9415%),
e) while on the other hand the dose reduction in the adhesive composition B towards the backing does not exceed 45%, preferably 25%, of the target dose.
The target dose denote s that radiation dose absorbed by the adhesive composition at which optimum product properties are obtained.
The acceleration voltage and the radiation dose for the first irradiation are then calculated in such a way (for example, with Gauss-Newton approximation) that the total radiation dose in the layer of adhesive composition A deviates by less than 30%, preferably less than 10%, from the desired target value, which may be different from the target value of the adhesive composition B.
As a basis for the calculation use is made, for example, of the following empirical formula, which was published by Neuhaus-Steinmetz at RadTech Europe, Mediterraneo 1993.       D    ⁢          xe2x80x83        [    %    ]    =            exp      ⁢              xe2x80x83            ⁢              {                  -                                    (                                                                    18.8                    *                    X                                                                              (                                              U                        B                                            )                                        1.57                                                  -                0.7                            )                        2                          }                    1      +                        (                                    9.7              *              X                                                      (                                  U                  B                                )                            1.57                                )                15            
where D is the dose in % UB is the acceleration voltage in kV X is the irradiated weight per unit area, in g/m2, consisting of the weights per unit area of the vacuum window, of the air gap between vacuum window and product, and of the depth in the product
The asymmetric irradiation from two sides permits a defined and sufficiently uniform radiation dose in the two adhesive layers with a greatly reduced radiation dose in the release liner. This does away with laborious and error-susceptible re-covering operations.
In addition, the process also permits controlled differences in the degree of crosslinking of the adhesive compositions on either side of the adhesive tape.
It is also possible to set controlled crosslinking profiles over the depth of the layers of adhesive composition. For this purpose, the acceleration voltages and set doses in the first and second irradiation are preferably chosen so that towards the backing in the adhesive composition A an ascending or descending depth/dose profile is formed, with which adhesive properties are influenced in a controlled manner.
The resulting radiation doses can, furthermore, preferably be up to 80 kGy or more in the adhesive compositions, and the chosen EB acceleration voltages can be from 40 to 350 kV. The weight per unit area in the beam path to the product, which is dependent on the design of an electron beam accelerator and consists of the weights per unit area of the vacuum window and of the air gap to the product, can typically be between 20 and 250 g/m2.
The adhesive compositions used can be acrylates and rubbers, applied from solution or from dispersion, or else hot-melt adhesive compositions, it also being possible for the adhesive compositions A and B to be different.
The adhesive compositions may, furthermore, be filled, coloured and/or foamed.
The backing materials employed are, in particular, those made of MOPP, BOPP, HDPE, LDPE, polyester, PVC, paper, nonwovens or foams.
Finally, the process can be employed in general for the radiation crosslinking of double-sidedly coated backing materials, where a double-sidedly coated backing material is irradiated asymmetrically from both sides with different doses in an irradiation means.
In this case, the coatings concerned are not adhesive compositions.
In a first alternative preferred embodiment, the process for the radiation crosslinking of double-sided adhesive tapes consists of the following steps:
a) coating a backing material with an adhesive A,
b) EB-crosslinking the adhesive A/backing subunit on the composition side with a set dose A and the acceleration voltage A, it being possible for the adhesive A to be both UV- and EB-crosslinkable,
c) lining the adhesive A with a release liner,
d) coating the second side of the backing material with the adhesive B, and
e) UV-irradiating the assembly on the exposed side of the adhesive composition B with a surface dose B.
For the first irradiation, the acceleration voltage and the set dose for the electron beams, and for the second irradiation the penetration depth of the UV radiation, can be chosen, through the selection of a photoinitiator with an appropriate wavelength, in such a way that the profile of crosslinking towards the backing in the adhesive composition A is virtually constant.
This embodiment also offers advantages with EB-degradable backings, which in this case are exposed significantly less than in the case of EB crosslinking from two sides. In addition, the release liner receives no EB dose.
In the second crosslinking pass with UV radiation, the absorbed UV dose in the adhesive B decreases with the depth towards the backing, approximately in an exponential function. Furthermore, it passes through a UV-permeable backing before penetrating into the adhesive A, where, in turn, an approximately exponential drop in the UV dose takes place from the backing towards the release liner.
In the first crosslinking pass with EB, the set dose A and the acceleration voltage A are chosen so that in the adhesive A following the second crosslinking pass with UV radiation an approximately uniform crosslinking over the depth is achieved, specifically by means of an opposite EB dose gradient, or else so that controlled profiles of crosslinking are produced.
If the precise depth-related decrease in the UV dose in the adhesives and in the backing is known, the parameters can be optimized in a purely mathematical procedure. Otherwise, experimental optimization, in particular of the accelerating voltage A, is advantageous.
In a second alternative preferred embodiment, the process for the radiation crosslinking of double-sided adhesive tapes consists of the following steps:
a) coating a backing material with an adhesive A, which can be both UV- and EB-crosslinkable,
b) UV-irradiating the backing, provided with the adhesive A, with a surface dose A in an irradiation means, it being possible for the adhesive A to be both UV- and EB-crosslinkable,
c) lining the adhesive A with a release liner,
d) coating the second side of the backing material with the adhesive B, and
e) EB-irradiating the assembly on the exposed side of the adhesive composition B with a dose B and acceleration voltage B set on the EB unit, the side carrying the release liner being guided through the EB unit preferably on a cooling roll.
Within the adhesive A, the absorbed UV dose decreases with the depth towards the backing in an approximately exponential function, the extent of the decrease in the desired wavelength range depending on various factors. From a certain layer thickness, therefore, there is insufficient crosslinking of the adhesive towards the backing. This is compensated by an opposite EB dose gradient from the EB irradiation in the following pass.
The set dose B and the acceleration voltage B are chosen in this case so that the dose ranges for the adhesive B and for the release liner, specified in the first advantageous embodiment, are observed. If the precise depth-related decrease of the UV dose is known, the parameters can be optimized by purely mathematical means. Otherwise, an experimental optimization (in particular of the accelerating voltage B) is advantageous.
In a third alternative preferred embodiment, the process for the radiation crosslinking of double-sided adhesive tapes consists of the following steps:
a) coating a release liner with an adhesive A,
b) EB-crosslinking the adhesive A/release liner subunit on the composition side with a dose A and acceleration voltage A set on the EB unit,
c) lining the adhesive A with a backing material,
d) coating the second side of the backing material with the adhesive B, and
e) EB-irradiating the assembly on the exposed side of the adhesive composition B with a dose B and acceleration voltage B set on the EB unit, the side carrying the release liner being guided through the EB unit preferably on a cooling roll and the dose A and the dose B and/or the acceleration voltage A and the acceleration voltage B having different values.
In a fourth alternative preferred embodiment, the process for the radiation crosslinking of double-sided adhesive tapes consists of the following steps:
a) coating a release liner with an adhesive A,
b) UV-irradiating the release liner, provided with the adhesive A, with a surface dose A in an irradiation means, it being possible for the adhesive A to be both UV- and EB-crosslinkable,
c) lining the adhesive A with a backing material,
d) coating the second side of the backing material with the adhesive B, and
e) EB-irradiating the assembly on the exposed side of the adhesive composition B with a dose B and acceleration voltage B set on the EB unit, the side carrying the release liner being guided through the EB unit preferably on a cooling roll and the surface dose A being different from the dose B.
For the first irradiation, the penetration depth of the UV radiation, through the selection of a photoinitiator with an appropriate wavelength, and for the second irradiation the acceleration voltage B and the set dose B for the election beams, can be chosen in such a way that the profile of crosslinking towards the backing in the adhesive composition A is virtually constant, with which adhesive properties are influenced in a controlled manner.
Furthermore, for the first irradiation, the penetration depth of the UV radiation can be chosen through the selection of a photoinitiator with an appropriate wavelength, and for the second irradiation the acceleration voltage B and the set dose B for the electron beams can be chosen in such a way that the profile of crosslinking towards the backing in the adhesive composition A is ascending or descending, with which adhesion properties are influenced in a controlled manner.
The purpose of the text below is to illustrate processes of the invention on the basis of a number of examples without, however, wishing to restrict the said processes unnecessarily.